Mass spectrometric detection/quantitation of peptides has traditionally required the precise selection of unique MRM transitions. However, in many instances, peptides may not fragment with high efficiency (especially cyclic peptides such as oxytocin), which can lead to additional sensitivity losses (e.g., during the MS/MS stage). Also, peptides can be difficult to quantitate when present in a crude sample (e.g., a biological sample) or a mixture containing other peptides, biological molecules, chemicals, proteins, lipids, etc. Though additional emphasis has been placed on improving sample preparation techniques (e.g., immunocapture, nanoLC, microLC), such techniques can increase cost, complexity, and/or throughput of the analysis, for example, due to the care required in column loading, equilibration time, and optimizing the flow rate.
Previous attempts at detection of peptides using differential mobility spectrometry have shown promise, however, as peptides can exhibit unique behaviors within a differential mobility spectrometer (DMS). For example, some peptides have the tendency to separate from singly charged chemical noise and separate based on the peptide's charge states. It has also been shown, however, that use of a chemical modifier in the transport region of a DMS can lead to alterations of the detected peptide states through proton stripping from the peptide and ultimately hinder detection of the peptide as a multiply charged ion. Accordingly, there remains a need for improved quantitation of peptides with enhanced discrimination between charged species.